Magnetic Resonance Images (MRIs)
A magnetic resonance image (MRI) offers an unparalleled view of the inside of a body, revealing both soft tissue structures and bone (as opposed to an x-ray or an ultrasound, each of which has its uses but also its limitation -- neither are considered adequate for showing if a dog has SM, because an X-ray can show the skull malformation, but often the malformation is too subtle for an x-ray, which also cannot show the fluid-pocket syrinxes; and an ultrasound, while potentially useful for a basic diagnosis when syrinxes are very noticeable, doesn't give adequate detail or a very clear image).
MRIs, because they are quite expensive, have only recently begun to be used in veterinary practice. Their use in this way has enabled researchers to finally see clearly what is happening in a dog's head when it has SM and for the first time also revealed the worrying extent of the skull malformation as well as SM in the breed. International research studies by different researchers across several continents have revealed a very high rate of affectedness, including in dogs showing no symptoms -- about 85-90% with the Chiari-like malformation, and from 30-70% incidence of SM in the various samples. The youngest (under age 5) sample had the lowest incidence.
Many researchers now feel there is a race against time to learn as much as possible about CM/SM in cavaliers to try and find the mode of inheritance and hopefully a genetic marker, so that breeders will know which cavaliers may be most prone to passing along genes that could cause more severe forms of syringomyelia. Find out about low cost MRI clinics for cavaliers here.
MRI images of affected and clear cavaliers
Nothing is more helpful for understanding what SM is and what it does inside the heads and spines of cavaliers than to view some MRIs.
Here is a link to a primer on how to read an MRI:
MRI primer for SM cavaliers
Some actual images:
MRI images of Cavaliers diagnosed without the malformation or SM
MRI images of Cavaliers diagnosed with the malformation but no SM
MRI images of Cavaliers diagnosed with the malformation and SM
MRI images of Cavaliers diagnosed with the malformation and moderate to severe SM
Two basic comparison images
Here is an image of a cavalier without SM. The skull has room for the brain, and the spinal cord (the cord that continues down the neck on the right of the image) is clear and solid:
Below is an image of a cavalier with moderate grade SM -- you can see the skull is smaller and the brain is compressed on the right and forced out the base of the skull. The spinal cord then has a characteristic kink where it gets squished and folded to accommodate the protruding brain (the malformation of the skull which cuts back in to the left, the protrusion and the kink are marked by the circle in the middle of the image). The spinal cord is not clear and you can see a syrinx, the dark pocket appearing in the middle of the cord (the second circle on the far right).
Of these two cavaliers, the first is considered not to have a very good head by show standards, while the second dog is considered to have an excellent head, which is smaller, more rounded and narrower than the first dog's skull, with a steeper drop at the back of the head. While it is unknown whether certain skull shapes increase the likelihood of the malformation and therefore of a dog having SM, some researchers have noted that the increase in SM cases has coincided with the preference for smaller skulls that have a more truncated appearance if viewed from the side.
A Swedish study published by a research student in Feb 2006 concludes that the bone malformation that causes SM coincides with skulls that are more steeply sloped at the back. The paper (in Swedish) may be downloaded here but you can also see an English abstract for the paper, which concludes: “This study showed that there is a difference in the shape of the caudal fossa between dogs with a normal head shape and small bred dogs with a steep back of the head. This indicates that occipital bone hypoplasia is related to a head shape where the back of the head is steep and that the malformation is common in these breeds”. The abstract notes that the malformation occurs in several breeds with skulls of this shape, but that SM primarily occurs in cavaliers.